In traditional analog-to-digital conversion, a signal is “blindly” converted without any consideration to what the signal type or statistics are. As a result, the conversion process is very power inefficient.
FIG. 1 illustrates a prior art quantization technique and its inefficiency in the amplitude and frequency domains. In traditional analog-to-digital conversion 100, the full voltage scale and Nyquist spectrum of the input is quantized. In the amplitude domain 101, quantization of the wasted space not occupied by a signal results in power inefficiency. The traditional ADC suffers from inefficient signal digitization in the frequency domain as well 102. This is because the signal is not always occupying the entire Nyquist spectrum band and, in most applications, is concentrated at certain frequency portions of this band.
FIG. 2 illustrates a prior art companding quantization principle. The known method of companding 200, frequently used for audio signals, helps make the quantization process more efficient by designing the quantization levels non-uniformly throughout the voltage full scale according to the known input signal. This method, however, suffers from difficulty in implementation of non-uniform quantization levels. Furthermore, once the non-uniform levels are designed, the ADC is only optimum for a known signal statistic, and not for time-varying statistics or a multitude of signal types.
Also, there is only amplitude compression in companding, and no frequency compression. Because the quantization levels are non-uniformly designed, some signal levels can suffer significantly more quantization noise compared to other signal levels, which may result in severe loss of information.
Performance parameters in ADCs such as speed, resolution, and power can be designed to be programmable. While these parameters can be reprogrammed during ADC startup or to account for low frequency drifts using low frequency digital adaptation, adaptation time can only be changed every few microseconds to milliseconds, and ADC power down or power up can also take just as long. Thus, such methods cannot optimize themselves to fast frequency variations. Furthermore, the ADC cannot be powered down during signal inactivity, because the ADC is unaware of when the next signal may be coming in.